The present application claims a priority from Japanese Patent Application No. 2015-220676, filed Nov. 10, 2015, which is incorporated herein by reference.
The present invention relates to a rotary carburetor for a two-stroke internal combustion engine and, more particularly, to a rotary carburetor preferably applicable to a stratified scavenging engine. The present invention typically preferably applied to a single cylinder engine mounted on a portable working machine such as a bush cutter, a chain saw, and a power blower.
A stratified scavenging engine is known as one form of two-stoke internal combustion engines (Patent Document 1). The stratified scavenging engine is characterized by a scavenging stroke. In the scavenging stroke of the stratified scavenging engine, a leading air is introduced into a combustion chamber in an early phase thereof. Subsequently, a fuel-air mixture pressurized in a crank chamber is supplied through a scavenging passage to the combustion chamber. Before the scavenging stroke, the scavenging passage is obviously filled with an air through a piston groove. The scavenging passage communicates with the combustion chamber and the crank chamber. The communication between the scavenging passage and the combustion chamber is opened and closed by a piston. The piston groove means a groove formed in a circumferential surface of the piston.
A large number of portable working machines are equipped with carburetors. A fuel-air mixture generated by a carburetor is supplied to a crank chamber of a piston-valve two-stroke engine. The carburetors are roughly classified into a butterfly type and a rotary type depending on a type of an output control valve. A butterfly carburetor includes a butterfly valve. A rotary carburetor includes a rotary valve.
Patent Document 2 discloses a stratified scavenging two-stroke engine and a rotary carburetor incorporated therein. The stratified scavenging two-stroke engine includes an intake system having on the downstream side of the carburetor a fuel-air mixture passage communicating with the crank chamber and an air passage communicating with an upper part of a scavenging passage through a piston groove disposed on a circumferential surface of a piston, and the fuel-air mixture passage and the air passage are sectioned by a dividing wall. The dividing wall extends to a main body of the rotary carburetor. The rotary carburetor is supplied with an outside air filtered by an air cleaner.
Therefore, the outside air supplied to the rotary carburetor goes through the rotary carburetor and is divided to the air passage and the fuel-air mixture passage. The air passage is supplied with the outside air. The scavenging passage is filled with the outside air through the air passage. On the other hand, the fuel-air mixture passage is supplied with the fuel-air mixture generated by the carburetor, and the crank chamber is supplied with the fuel-air mixture through the fuel-air mixture passage.
The rotary carburetor disclosed in Patent Document 2 has a carburetor main body and a columnar rotary valve. The rotary valve has a through-hole with a circular cross section making up a gas passage. This through-hole extends in a direction orthogonal to an axis of the rotary valve. The carburetor main body rotatably accepting the rotary valve has an inlet and an outlet. The inlet of the carburetor main body is made up of a circular opening and the diameter of the circular opening is the same as the diameter of the through-hole of the rotary valve. The outlet of the carburetor main body is sectioned by a body dividing wall into two ports. A first port communicates with the air passage. A second port communicates with the fuel-air mixture passage. The air passage and the fuel-air mixture passage of the intake system are arranged in the extending direction of an axis of an engine cylinder. Describing by using terms “upper” and “lower” based on the top dead center and the bottom dead center of the engine, the air passage is located on the upper side and the fuel-air mixture passage is located on the lower side thereof.
The rotary carburetor disclosed in Patent Document 2 has a nozzle tube coaxial with the rotation axis of the rotary valve as is the case with the conventional carburetor. Fuel is supplied from this nozzle tube into the through-hole. The nozzle tube is fixed to the carburetor main body. Therefore, the rotary valve rotates relatively to the stationary nozzle tube. The rotary carburetor disclosed in Patent Document 2 has the rotation axis of the rotary valve positioned sideways. Therefore, the rotation axis of the rotary valve has an arrangement relation orthogonal to the axis of the engine cylinder. A fuel outlet opened in a circumferential wall of the nozzle tube is oriented to the second port of the carburetor main body, i.e., the port communicating with the fuel-air mixture passage. Describing by using the terms “upper” and “lower”, the fuel outlet of the nozzle tube is oriented toward the lower side and the fuel discharged from this fuel outlet is directed through the second port of the carburetor main body to the fuel-air mixture passage.
A rotary carburetor of Patent Document 3 has a rotation axis of a rotary valve positioned sideways as is the case with the rotary carburetor of Patent Document 2. The rotary carburetor of Patent Document 3 includes a nozzle tube having a guide tube portion extending downward with a slope, and the fuel discharged from the fuel outlet is guided through this guide tube portion to the second port of the carburetor main body. Therefore, the fuel discharged from the fuel outlet of the nozzle tube is guided by the guide tube portion in an integral structure with the nozzle tube and is directed to the second port of the carburetor main body.